International Assessment of Research and Development on Carbon Nanotubes: Manufacturing and Applications [1st edition]

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WTEC Panel Report on

INTERNATIONAL ASSESSMENT OF RESEARCH AND DEVELOPMENT OF CARBON NANOTUBE MANUFACTURING AND APPLICATIONS

Peter Eklund (Panel Chair) Pulickel Ajayan Robert Blackmon A. John Hart Jing Kong Bhabendra Pradhan Apparao Rao Andrew Rinzler

World Technology Evaluation Center, Inc. 4800 Roland Avenue Baltimore, Maryland 21210

WTEC PANEL FOR R&D OF CARBON NANOTUBE MANUFACTURING AND APPLICATIONS Sponsored by the National Science Foundation, the Office of Naval Research, the National Institute of Biomedical Imaging and Bioengineering, and the National Institute of Standards and Technology. Peter Eklund (Panel Chair) Dept. of Materials Science and Engineering Pennsylvania State University University Park, PA Pulickel Ajayan Dept. of Materials Science and Engineering Rensselaer Polytechnic Institute Troy, New York Robert Blackmon Harper International Lancaster, NY A. John Hart Dept. of Electrical Engineering and Computer Science Massachusetts Institute of Technology Cambridge, MA

Jing Kong Dept. of Electrical Engineering and Computer Science Massachusetts Institute of Technology Cambridge, MA Bhabendra Pradhan Columbian Chemicals Company Marietta, GA Apparao Rao Dept. of Physics and Astronomy Clemson University Clemson, SC Andrew Rinzler Dept. of Physics The University of Florida Gainesville, FL

WTEC WTEC provides assessments of international research and development in selected technologies under awards from the National Science Foundation (NSF), the Office of Naval Research (ONR), and other agencies. Formerly part of Loyola College, WTEC is now a separate nonprofit research institute. Michael Reischman, Deputy Assistant Director for Engineering, is NSF Program Director for WTEC. Sponsors interested in international technology assessments and related studies can provide support for the program through NSF or directly through separate grants or GSA task orders to WTEC. WTEC’s mission is to inform U.S. scientists, engineers, and policymakers of global trends in science and technology. WTEC assessments cover basic research, advanced development, and applications. Panels of typically six technical experts conduct WTEC assessments. Panelists are leading authorities in their field, technically active, and knowledgeable about U.S. and foreign research programs. As part of the assessment process, panels visit and lead extensive discussions with foreign scientists and engineers in their labs. The WTEC staff helps select topics, recruits expert panelists, arranges study visits to foreign laboratories, organizes workshop presentations, and finally, edits and disseminates the final reports.

WTEC Panel Report on

INTERNATIONAL ASSESSMENT OF CARBON NANOTUBE MANUFACTURING AND APPLICATIONS FINAL REPORT

June 2007

Peter Eklund (Panel Chair) Pulickel Ajayan Robert Blackmon A. John Hart Jing Kong Bhabendra Pradhan Apparao Rao Andrew Rinzler

This document was sponsored by the National Science Foundation (NSF) and other agencies of the U.S. Government under an award from NSF (ENG-0423742) to the World Technology Evaluation Center, Inc. The Government has certain rights in this material. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the United States Government, the authors’ parent institutions, or WTEC, Inc.

Abstract This WTEC study focuses on the manufacturing and applications of carbon nanotubes (CNTs) to identify recent progress in understanding the commercial potential of CNTs as viewed by academic, industrial, and government research facilities around the world. CNT manufacturing methods and equipment, processing and separation techniques, characterization procedures, and opportunities for international collaboration are highlighted in this study. These issues are also discussed in the context of leading electronic, optical, and mechanical applications of CNTs ranging from transistors to structural composites. CNTs can be produced by many methods, and depending on the diameter, one can obtain either single-walled CNTs (SWCNTs) or multi-walled CNTs (MWCNTs). The current capacity for the production of MWCNTs far exceeds that of SWCNTs. SWCNTs are much more expensive and difficult to manufacture than MWCNTs, and there is not yet a distinct large-scale market for SWCNTs, which is needed to drive down the production cost. For both types of CNTs, Asia’s production capacity is two to three times higher than that estimated for North America and Europe combined; Japan is the prominent leader in the production of MWCNTs. Use of CNTs in lithium-ion battery electrodes is the current driving force of ton-scale MWCNT production in Japan. CNT-replacement products for indium tin oxide (ITO) and field emission devices (FEDs) are driving increased production of SWCNTs, whereas applications using transistors require precise control over CNT diameter and conductivity, which is farther from commercial realization. When the cost of bulk SWCNTs decreases significantly, applications in electromagnetic shielding (EMI) and electrostatic discharge (ESD) protection can be expected, and SWCNTs will replace MWCNTs in conductive plastics. Currently, owing to challenges in CNT functionalization and dispersion, CNTs can only achieve modest enhancements in mechanical properties of materials. While sporting goods with CNT reinforcement are beginning to appear on the market, improved processing techniques along with manufacturing techniques for long and aligned CNTs (e.g., yarns and aligned arrays) are needed to realize CNT-based materials that provide compelling advantages over existing fiber composites in lightweight structures.

World Technology Evaluation Center, Inc. (WTEC) R.D. Shelton, President Michael DeHaemer, Executive Vice President Geoffrey M. Holdridge, Vice President for Government Services David Nelson, Vice President for Development Y.T. Chien, Senior Fellow Hassan Ali, Director of International Study Operations Maria L. DeCastro, Director of Publications, Editor Masanobu Miyahara, Advance Contractor (Japan) Gerald Hane, Advance Contractor (China and Korea)

Copyright 2007 by WTEC, Inc. The U.S. Government retains a nonexclusive and nontransferable license to exercise all exclusive rights provided by copyright. Some WTEC final reports are distributed by the National Technical Information Service (NTIS) of the U.S. Department of Commerce. A list of available WTEC reports and information on ordering them from NTIS is on the inside back cover of this report.

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FOREWORD

We have come to know that our ability to survive and grow as a nation to a very large degree depends upon our scientific progress. Moreover, it is not enough simply to keep 1 abreast of the rest of the world in scientific matters. We must maintain our leadership. President Harry Truman spoke those words in 1950, in the aftermath of World War II and in the midst of the Cold War. Indeed, the scientific and engineering leadership of the United States and its allies in the twentieth century played key roles in the successful outcomes of both World War II and the Cold War, sparing the world the twin horrors of fascism and totalitarian communism and fueling the economic prosperity that followed. Today, as the United States and its allies once again find themselves at war, President Truman’s words ring as true as they did a half-century ago. The goal set out in the Truman Administration of maintaining leadership in science has remained the policy of the U.S. Government to this day: Dr. John Marburger, the Director of the Office of Science and Technology (OSTP) in the Executive Office of the 2 President made remarks to that effect during his confirmation hearings in October 2001. The United States needs metrics for measuring its success in meeting this goal of maintaining leadership in science and technology. That is one of the reasons that the National Science Foundation (NSF) and many other agencies of the U.S. Government have supported the World Technology Evaluation Center (WTEC) and its predecessor programs for the past 20 years. While other programs have attempted to measure the international competitiveness of U.S. research by comparing funding amounts, publication statistics, or patent activity, WTEC has been the most significant public domain effort in the U.S. Government to use peer review to evaluate the status of U.S. efforts in comparison to those abroad. Since 1983, WTEC has conducted over 60 such assessments in a wide variety of fields, from advanced computing, to nanoscience and technology, to biotechnology. The results have been extremely useful to NSF and other agencies in evaluating ongoing research programs, and in setting objectives for the future. WTEC studies also have been important in establishing new lines of communication and identifying opportunities for cooperation between U.S. researchers and their colleagues abroad, thus helping to accelerate the progress of science and technology generally within the international community. WTEC is an excellent example of cooperation and coordination among the many agencies of the U.S. Government that are involved in funding research and development: almost every WTEC study has been supported by a coalition of agencies with interests related to the particular subject at hand. As President Truman said over 50 years ago, our very survival depends upon continued leadership in science and technology. WTEC plays a key role in determining whether the United States is meeting that challenge, and in promoting that leadership. Michael Reischman Deputy Assistant Director for Engineering National Science Foundation

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Remarks by the President on May 10, 1950, on the occasion of the signing of the law that founded the National Science Foundation. Public Papers of the Presidents 120: 338. 2

http://www.ostp.gov/html/01_1012.html.

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iii TABLE OF CONTENTS Foreword .............................................................................................................................................................i Table of Contents ..............................................................................................................................................iii Executive Summary .........................................................................................................................................vii 1.

Introduction Peter C. Eklund Background and Scope ........................................................................................................................ 1 Methodology........................................................................................................................................ 2 Overview of the Report........................................................................................................................ 5

2.

Scaling Up of CNT Production, Separation, and Purification Robert Blackmon and Andrew Rinzler Background.......................................................................................................................................... 7 State-of-the-Art in CNT Production: Capacity .................................................................................... 7 State-of-the-Art in CNT Production: Techniques ................................................................................ 8 Patents................................................................................................................................................ 11 Gaps between Existing Knowledge and Required Capabilities ......................................................... 13 Summary and Conclusions ................................................................................................................ 15 References ......................................................................................................................................... 17

3.

Dispersion, Functionalization, and CNT Blends Bhabendra Pradhan Background........................................................................................................................................ 19 Functionalization ............................................................................................................................... 25 Observations ...................................................................................................................................... 27 References ......................................................................................................................................... 27

4.

Ordered Arrays of Carbon Nanotubes Apparao M. Rao Background........................................................................................................................................ 29 Summary............................................................................................................................................ 33 References ......................................................................................................................................... 34

5.

Mechanical Applications of Carbon Nanotubes Pulickel M. Ajayan Background........................................................................................................................................ 35 Mechanical Properties of Carbon Nanotubes..................................................................................... 35 Mechanical Applications of Carbon Nanotubes ................................................................................ 36 Carbon Nanotubes in Advanced Composites..................................................................................... 36 Challenges and Future Prospects ....................................................................................................... 42 References ......................................................................................................................................... 42

6.

Electronic, Optical, and Optoelectronic Applications of Carbon Nanotubes A. John Hart, Andrew Rinzler, and Jing Kong Background........................................................................................................................................ 45 Promises of CNT electronics ............................................................................................................. 45 Electronics-Oriented CNT Manufacturing and Major Challenges..................................................... 47 Leading Electronic and Optoelectronis Applications Of CNTs......................................................... 49 References ......................................................................................................................................... 61

iv Appendices A.

Appendix A. Panelist Biographies.................................................................................................. 65

B.

Appendix B. Site Reports Advanced Industrial Science and Technology (AIST) ...................................................................... 70 Institute of Physics, Chinese Academy of Sciences (IOP-CAS) ....................................................... 72 Kaneko Research Lab........................................................................................................................ 75 Iljin Nanotech .......................................................................................................................... 75 Nano Carbon Technologies Co., Ltd. (NCT)..................................................................................... 81 National Center for Nanoscience and Technology (NCNST) ........................................................... 84 NEC Corporation (NEC) ................................................................................................................... 86 Center for Nanoscale Science and Technology ................................................................................. 88 The Institute of Physical and Chemical Research ............................................................................. 93 Samsung SDI Co., Ltd....................................................................................................................... 95 NANO Systems Institute (NSI) ......................................................................................................... 97 Shenzhen Nano-Technologies Port Co., Ltd. (NTP) ......................................................................... 99 Shinshu University .......................................................................................................................... 101 Showa Denko (SDK)....................................................................................................................... 104 Sun Nanotech Co., Ltd. ................................................................................................................... 106 Tsinghua University- Tsinghua-Foxconn Nanotechnology Research ............................................. 109 Beijing Key Lab of Green Chemical Reaction Engineering and Technology ................................. 113 Yonsei Nanomedical- National Core Research Center.................................................................... 116

C.

Appendix C. Glossary ................................................................................................................... 118

v LIST OF FIGURES 2.1 Production system at Sun Nanotech...........................................................................................................9 2.2 Production system at NTP..........................................................................................................................9 2.3 CVD-based quartz tube CNT units at research laboratories at Peking University...................................10 2.4 CVD-based quartz tube CNT units at Tsinghua-FoxConn Nanotechnology Research Center. ...............10 2.5 CVD based fluid bed unit in stainless steel materials at Tsinghua University. ........................................11 2.6 HRTEM images of bulk and high-purity grade MWCNTs......................................................................12 2.7 Pilot plant production system at SouthWest Nanotechnologies...............................................................13 2.8 HiPCO process, Carbon Nanotechnologies, Inc. (CNI). ..........................................................................13 2.9 RF Plasma process, University of Sherbrooke. ........................................................................................13 2.10 International patent of CNT filings and issuances. ..................................................................................15 3.1 Agglomeration of carbon nanomaterials is a problem. ............................................................................20 3.2 Example of poor and good dispersion of carbon nanomaterials in solution.............................................21 3.3 Example of poor vs. good dispersion of carbon nanomaterials in polyurethane films, and demonstration of stretching of well-dispersed CNT-polyurethane film...................................................21 3.4 Process developed for dispersion of CoMoCat nanotube by SWNeT......................................................22 3.5 Schematic of floating catalyst CVD method............................................................................................22 3.6 Microstructure of MWCNT materials produced by a continuous process at Nano Carbon technologies Co., Ltd, Japan. ...................................................................................................................23 3.7 Scanning electron microscope (SEM) image of cross section of 5% MWCNTs in polycarbonate matrix, Nano Carbon Technologies Co, Ltd, Japan. ................................................................................24 3.8 Electrical percolation in SWNT-polymer composites..............................................................................24 3.9 Zyvex’s functionalization technology. .....................................................................................................25 3.10 Integration of the nanotubes into the polymer cross-linked structure. .....................................................26 3.11 TEM images with EDS for purified SWCNTs and Fc–SWCNTs adduct; purified SWNT solely; Fc–SWCNTs in DMF. .............................................................................................................................27 4.1 Scanning electron microscope (SEM) images of self-oriented MWCNT towers synthesized silicon substrates; SEM images showing sharp edges and corners at the top of a nanotube tower; wellaligned MWCNTs in the direction perpendicular to the substrate surface...............................................29 4.2 Vesselin Shanov and coworkers at University of Cincinnati’s Smart Materials Nanotechnology Laboratory used plasma enhanced CVD to grow 7 mm tall MWCNT arrays on Si substrates; an expanded view of the as-grown MWCNT array image............................................................................30 4.3 A schematic of Fujitsu’s vision for integrating vertically aligned MWCNTs as vias for large-scale integrated circuits.....................................................................................................................................30 4.4 Large area oriented MWCNTs prepared using Fujitu’s low temperature growth process involving hot filament/dc plasma CVD method and Ni and Co catalysts. ...............................................................31 4.5 Interconnects for LSI are one of the target applications in Fujitsu's nano technology program. .............31 4.6 Oriented arrays of SWCNTs prepared using the super growth method. ..................................................32 4.7 A schematic of the template-assisted CVD process developed by Toray Industries, Japan.....................33 4.8 SEM images of individual SWCNTs prepared on an oxidized Si surface. ..............................................33 5.1 5.2 5.3 5.4 5.5

SEM images showing typical dispersion of nanotubes inside a polymer matrix. ...................................37 MWCNT based polymer sporting goods..................................................................................................38 Nanotubes incorporated into 3D fiber laminae reinforced polymer composites. .....................................39 Mechanism for long cycle ability.............................................................................................................41 Cyclic characteristics of synthetic graphite anodes in Li ion batteries as a function of weight percentage of MWCNT in the anode.. .....................................................................................................41

6.1 Map of prospective electronic applications of CNTs, relating each application area to the required complexity of CNT manufacturing and integration. ................................................................................46 6.2 Arc-grown MWCNT showing the near perfect wall graphitization..........................................................47

vi 6.3 CVD-grown MWCNTs showing the disorder in the wall structure typical of MWCNT CVD synthesis................................................................................................................................................... 48 6.4 A CVD-synthesized MWCNT following a 2600oC heat treatment to graphitize its walls. ..................... 48 6.5 Variation of on-current of a CNT-FET as a function of SWNT diameter with three different contact metals. ......................................................................................................................................... 49 6.6 Electrical percolation in SWNT-polymer composite, reached at 0.062% of “long” CNTs made by arc discharge using a Ni/Y catalyst; EMI shielding effectiveness of SWNTs and MWCNTs................. 50 6.7 Aggregates of single-wall carbon nanohorns coated with ~2 nm diameter Pd nanoclusters attached using poly(vinylpyrrolidone). .................................................................................................................. 52 6.9 Schematic of CNT x-ray emission source; x-ray and computed tomography images of a computer chip and a mouse carcass obtained using CNT-based x-ray source......................................................... 54 6.10 Concept and prototype schematic of nonvolatile CNT memory device, using bistable electromechanical junctions of suspended CNTs. ................................................................................... 56 6.11 Integration and performance of vertical CNTs as microelectronic interconnects. ................................... 57 6.12 Typical architecture of a chemical sensor................................................................................................ 57 6.13 Scanning electron microscope image of a five-stage CNT ring oscillator circuit; voltagedependent frequency spectra.................................................................................................................... 59 6.14 The split gate device where Vg1 and Vg2 are biased with opposite polarities; I-V characteristics ......... 59 6.15 Light emission can be translated between the two metal electrodes by varying the gate voltage; schematic structure used to generate localized light emission ................................................................. 60 LIST OF TABLES 1.1 Panel Members ............................................................................................................................................ 2 1.2. Speakers and Presentations at the North American Baseline Workshop .................................................... 3 1.3 Speakers and Presentations at the Japan Workshop..................................................................................... 4 1.4 Speakers and Presentations at the China Workshop .................................................................................... 4 1.5 Sites Visited in Asia..................................................................................................................................... 5 2.1. Estimated Worldwide MWCNT Production Capacity, 2006...................................................................... 7 2.2 Estimated Worldwide SWCNT Production Capacity, 2006 ....................................................................... 8 2.3. Patent filings by Iljin Nanotech ................................................................................................................ 11 2.4. Asia and Europe: Summary of the Current Production Capacity and Techniques Available ................... 16 2.5. North America: Summary of the Current Production Capacity and Techniques Available...................... 17 2.6 Geographic Totals...................................................................................................................................... 17 5.1 Mechanical Properties of Different Materials for Comparison with Carbon Nanotubes ........................... 36 5.2. Properties Improved by Adding MWCNT................................................................................................ 38 6.1. Major Electrical, Optical, and Optoelectronic Applications of CNTs ...................................................... 46

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EXECUTIVE SUMMARY Peter C. Eklund

INTRODUCTION AND BACKGROUND In March 2006, the National Science Foundation (NSF), the Office of Naval Research (ONR), the National Institute of Standards and Technology (NIST), and the National Institute of Biomedical Imaging and Bioengineering (NIBIB) of the National Institutes of Health (NIH) requested the World Technology Evaluation Center (WTEC) to make an international assessment of the manufacturing and applications of carbon nanotubes (CNTs). The purpose was to gather and disseminate information on the worldwide status and trends in CNT manufacturing and applications. A panel of eight experts from universities and industry was organized and chaired by Professor Peter Eklund from Pennsylvania State University. The panel first held a North American Workshop on June 2, 2006, to gather information on CNT activity in Canada and the United States. The WTEC CNT Panel then traveled in the fall to Korea, China, and Japan to survey the primary activity in Asia. Carbon nanotubes appear to have been discovered in the late 1970s by Professor Morinobu Endo during his experiments to produce micron diameter carbon fibers (Oberlin 1976). During this time he published several transmission electron microscope (TEM) images showing very small diameter carbon filaments, which he described as a tube formed by rolling up a plane of carbon atoms as found in graphite. Surprisingly, Endo’s discovery passed largely unnoticed until very recently. In 1991, some fifteen years later, articles such as “Helical Microtubules of Graphitic Carbon,” published in Nature by Iijima (1991), demonstrated that CNTs could be produced in an arc discharge between carbon electrodes in He gas. Small quantities of CNTs then became available for study. Iijima and co-workers used this material and modern transmission electron microscopes to provide evidence for the structure of CNTs that we know today. They found that the CNTs could be open-ended, or closed with a hemispherical cap resembling half of a fullerene molecule. The work of Iijima and his coworkers resulted in a “feeding frenzy” by researchers worldwide to learn how to produce CNTs in larger quantities and of better quality. The race was on to elucidate their properties and to develop new technology taking advantage of the unique properties of these atomic filaments. Endo, in his early review of vapor-phase-grown carbon fibers (VPCF), also reminds us that he has observed a hollow tube, linearly extended with parallel carbon layer faces near the fiber core.(Endo 1988). This appears to be the observation of multi-walled carbon nanotubes at the center of the fiber. This work by Endo also contains several relatively unknown and very early references to the growth of carbon whiskers by R.J. Bacon and the earliest carbon fibers reported in 1890. The mass- produced MWCNTs (Japan) today are strongly related to the VPGCF developed by Professor Endo. In fact they call it the “Endo-process”, out of respect for his early work and patents. In fact, these MWCNTs are really small diameter VPGCFs subsequently subjected to a high temperature heat treatment (HTT) in an inert atmosphere. This HTT drives the structure into a more organized set of concentric tubes of carbon. The manufacture of these MWCNTs is described later in the report. The covalent carbon-carbon bonds within a CNT are among the strongest known. In addition, CNTs are remarkably compliant, i.e., they are able to make sharp bends without fracture. It is therefore expected that CNTs could be incorporated in revolutionary lightweight composites. To perfect these CNT composites, challenges exist in their dispersion into the host and also in developing strong, covalent coupling of the CNT to the host medium. The electronic structure of a CNT is also remarkable. The nature of the electrical conductivity along the tube axis was predicted theoretically to depend uniquely on the integers (n, m). These predictions were soon confirmed by experiment. Energy band theory predicted that two-thirds of all possible CNTs would be

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semiconducting with a gap Eg ~ 0.6 eV/d, where d is the diameter of the tube (Dresselhaus, Dresselhaus, and Avouris 2001). The remaining one-third of all CNTs should be metallic due to band overlap. Quite remarkably, several research groups soon found ballistic transport in CNTs at cryogenic temperatures, indicating that it was possible for electrons to pass distances ~100 nm down the tube without scattering. This result is testimony to the quasi one-dimensional nature of these filaments (carrier scattering is suppressed in one-dimensional systems) and to the possibility of producing reasonably perfect tubes (without too many wall defects). The high mobility of electrons and holes in CNTs were envisioned as a property to be exploited in the fabrication of high-speed semiconducting devices, possibly providing an extension of Moore’s law for the time evolution of circuit density in VLSI circuits. Larger diameter multi-walled CNTs have also been recently proposed as interconnects in VLSI circuits. Progress in the development of applications for CNTs in electronics has been encouraging. CNTs can be produced by many methods. They all require the presence of hot transition metal based nanoparticles (e.g., Fe,Co,Ni and/or their alloys) that act as “seeds” for growth. These particles (sometimes referred to as “catalysts”) are placed in contact with a gaseous carbon or hydrocarbon source that deposits carbon on the particle surface. The deposited C atoms then rapidly diffuse toward the base of the tube which grow from the catalyst particle surface. Many of the details of the growth process still need to be elucidated. Growth rates of ~1-10s microns/sec. have been reported. Depending on the nanoparticle diameter, one obtains either an individual single-walled CNT, or SWCNT, orfor larger particle diameters, one can obtain either a bundle of tightly packed SWCNTs or an altogether different structure consisting of a concentric set of tightly nested SWCNTs. In this case, the filament is called a multi-walled CNT, or a MWCNT. The spacing between the walls in a MWCNT is close to the basal plane separation in graphite (~0.34 nm). However, the concentric tube walls are not chemically bonded to one another. Rather, they are weakly coupled by the van der Waals force, so it is possible for one tube to slip within another without structural damage. Finally, MWCNTs can be grown most easily, and they can exhibit many cylindrical shells (as many as ~50) with an inner diameter as small as 1-2 nm and lengths in the tens of millimeters. MWCNTs, as we will learn shortly, are the first CNT to really make it to the marketplace. This success stems in part from the fact that they are easier to grow and in part from the fact that they have been in the R&D process for an additional 10-15 years relative to the SWCNT. It should be kept in mind that almost all the imaginable applications of CNTs attempt to take advantage of (1) their low mass density, small diameter, and high aspect ratio, (2) their high electron/hole mobility, (3) their high thermal conductivity, and (4) their unusual mechanical properties, i.e., high Young’s modulus, fracture toughness, and compliance. FINDINGS Large-Scale Production of Bulk CNTs Based on data collected as part of this study, we find that the current capacity for the production of MWCNTs far exceeds that of SWCNTs. MWCNT capacity is estimated at ~300 tons/yr, compared to ~7 tons/yr for SWCNTs. It should also be noted that the capacity for SWCNTs is far less certain than for MWCNTs and is based on more generalizations, e.g., the amount that might be produced if demand were higher, etc. Most of the production capacity for CNTs is derived from chemical vapor deposition (CVD) methods that use a hydrocarbon gas (e.g., methane) as the carbon feedstock; the catalyst bed in CVD can be either static or fluidized, and the catalyst particle size largely determines the product (e.g., SWCNT or MWCNT). However, a few companies provide SWCNTs produced by pulsed laser vaporization, radio frequency (RF) plasma, or electric arc discharge. Sources in North America appear to have focused more on the production of SWCNTs, primarily by CVD, but also by pulsed laser vaporization, RF plasma, and electric arc discharge. For both types of CNTs, Asian production capacity is two to three times higher than that estimated for North America and Europe combined. Japan is the clear leader in the production of MWCNTs, with three large companies (Showa Denko, Mitsui, and Hodogaya Chemical) backing the start-up of spin-off companies whose MWCNT production is forecast to exceed 100 tons/yr in the near term. Their MWCNT products appear to differ only in the diameter of the CNT; both companies use a process derived from the “Endo” CVD process, which has evolved considerably over 15 years in Japanese universities through government-

Executive Summary

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and industry-funded research programs. It should also be emphasized that the Japanese approach to largescale production of MWCNTs is a continuous (not a batch) process, with the final step involving a hightemperature heat treatment in excess of 2000°C. This treatment eliminates most of the catalyst (by vapor transport) and transforms a rather disorganized carbon filament (called a vapor grown fiber) into a wellordered MWCNT. In North America, there is only one large producer of MWCNTs, Hyperion; it produces approximately 50 tons/year. Hyperion’s tubes do not appear to receive a high-temperature heat treatment and are only sold in master batches, i.e., dispersed in monomer or polymer. Production of Carbon Nanotubes on Substrates Both SWCNTs and MWCNTs can be grown directly on substrates by the CVD process by first depositing the growth seeds on the substrate. Several electronics applications, e.g., field effect transistor arrays, field emission sources, and routes to distribute current to various layers of circuits in VLSI chips, are possible using the CNTs where they are grown. In a few cases, R&D is underway to explore the production of large quantities of CNTs (bulk production) by CVD growth from substrates. It was shown early-on that patterned CNT growth could be achieved on Si substrates by selectively removing the oxide. Another approach, of course, is by depositing the catalyst particles only where tubes are desired to be grown. If the catalyst particle number density is sufficiently high, a forest of self-supporting CNTs can be produced (Hata et al. 2004). With carefully controlled growth conditions, the CNT lengths can be 2-10 millimeters. Fujitsu (Japan) is perhaps the world’s leading producer of substrate-supported CNT products. In late 2005, it reported the first CNT-based heat sink technology for semiconductor chips (uniform growth over a ~10 inch diameter wafer). It is also involved in the patterned growth of CNT for very large-scale integration (VLSI) (the CNT can support 109 A/cm2 without undergoing degradation due to electromigration). Large-Scale Dispersion of CNTs To realize superior mechanical properties of CNT composites, it is essential to be able to uniformly disperse CNTs in the host matrix. The first step in the dispersion process is the appropriate functionalization of the outer CNT wall by adding a molecule that has a strong coupling to the intended solvent. CNT functionalization and dispersion has been a topic of considerable university and industrial research activity worldwide over the last ten years. This research has led to significant intellectual property development. To date, only one company, Zyvex, Inc. (North America), primarily focuses on the development and implementation of technology to disperse nanotubes. CNT dispersions are available, however, from a few CNT manufacturers worldwide. Several companies, e.g., Hyperion, will only transport CNTs in solution. Mechanical Applications of CNTs Experiments in the late 1990s indicated that CNTs have a Young’s modulus of 1 TPa, i.e., five times larger than the best steel and also five times larger than high-quality carbon fibers. TEM images have shown that CNTs also exhibit remarkable compliance, i.e., the ability to withstand large angle bending without breaking. Because of these remarkable mechanical properties (and low mass density), considerable R&D has been invested in the development of CNT composites, primarily (but not exclusively) with CNT-polymer composites. The incorporated CNTs can also provide electromagnetic shielding and electrostatic discharge benefits. Because of the lower cost and wider availability, MWCNT-polymer composites are now evolving into marketable products.. The primary application for MWCNTs is now in mechanically stabilizing carbon electrodes for Li-ion batteries. In this case, the CNT additive at 5-10 wt% provides a remarkable improvement in the number of charge-discharge cycles before mechanical failure occurs. The nanotubes combat material fatigue encountered during the expansion (charging) and contraction (discharging) as the Li is moved into and out of the carbon electrode. This application alone is forecast to require several hundred tons/yr of MWCNTs in the next few years. The WTEC team observed that the primary Japanese producers of MWCNTs are now in the process of doubling their current production capacity. Unfortunately, to date, the enhancement of the mechanical properties of the CNT composites attributable to the CNT additive is modest, e.g., the measured tensile strength improves 10-20% and Young’s modulus might double but still be 200 times less than that of the CNT itself. Nevertheless, sporting goods with low wt% CNT additions are starting to appear in the marketplace in Japan and the United States. It is not yet clear how the performance of the ski, tennis racket, baseball bat, or golf club is altered or enhanced.

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Peter C. Eklund

It is also clear that significantly longer CNTs, properly functionalized to covalently bond to the medium, will be needed to use their remarkable mechanical properties. This extra length would allow the fabrication of high modulus yarn, leading to woven CNT fabrics. If this extra length could be achieved, one could envision a large market for CNT composites in military and aerospace applications. A perhaps unexpected application of a CNT polymer for use as a catheter was previewed to the WTEC panel by Professor Endo during the panel’s visit to Shinshu University (Nagano, Japan). Here the unexpected property enhancement through CNT addition was in the surface smoothness and reduced friction of the flexible CNT-polymer catheter. During the panel visit, Prof. Endo also demonstrated that the incorporation of small amounts of CNTs into aluminum alloys dramatically enhanced their thermal conductivity. It seems likely that emerging limited markets will be available soon for both of these applications. Electronic, Optical, and Optoelectronic Applications CNTs are being investigated for commercial use in a variety of applications in electronic, optic, and optoelectronic devices. We can divide these applications somewhat artificially by the amount of CNTs required per device. Requiring relatively large amounts of CNTs (milligrams to tens of grams) are Field Emission Devices (FED); composites for electromagnetic interference (EMI) shielding and electrostatic discharge (ESD) protection; additives for battery and supercapacitor electrodes (their role here might only be in enhancing the mechanical stability of the electrodes); and fuel cell electrodes and membranes and transparent, flexible, and electrically conductive screens (e.g., “touch” screens). We have already mentioned that the Li-ion battery electrode application is currently a commercial reality and a success story for CNTs. It is the primary market driving force for the current increase in MWCNT production capacity in Japan. We suspect that CNT-based supercapacitors will soon become available and will drive the capacity even higher. Team members were unable to secure any significant details on this new market entry, however. Samsung (Korea) has made a substantial investment in developing large-area FEDs based on a forgiving process involving the application of a conductive CNT polymer paste on the back electrode and a subsequent lithographic process to produce the pixels. In this approach, bulk CNTs are used and blended with a polymer to form the paste. There seems to be some uncertainty in the literature as to which CNT is better for FEDs, i.e., SWCNTs, double-walled CNTs (DWCNTs) or MWCNTs. Apparently there is some disagreement over the trade-off between emission efficiency at low device bias versus emission lifetime. There is also a large market need for cheaper and better transparent electrodes to replace deposited thin films of indium tin oxide (ITO). Several, small start-up companies in North America are developing CNT replacement products for ITO. Flexible and transparent touch screens can be produced by depositing welldispersed SWCNTs on transparent plastic sheets. Together with FED devices, touch screens may be among the first high-volume applications of bulk SWCNTs. On the other end of the CNT electronics applications spectrum are devices that require micrograms or less of CNTs. Examples include field effect transistors (FETs) and FET arrays, FET-based or interdigitated resistive chemical and biosensors, and finally, circuit interconnects for VLSI chips (vias). In these cases, it is possible (or required) either to grow the CNTs in place on the substrate or deposit them after growth. For sensors to be “selective” to particular analytes, the nanotubes must either be functionalized to link with specific analytes or be covered with a membrane that passes only the selected analytes. Another application for individual CNTs is high-density logic arrays. Nantero is developing these arrays using individual SWCNTs as electromechanical nanoscale random access memory (NRAM) switches that can be held at low power in the “off” or “on” state. Surprisingly, these switches can change state in a few nanoseconds; Nantero claims that it has the technology to deliver these logic gates at high density, and that they can be integrated with complementary metal oxide semiconductor (CMOS) technology. CNT electronic devices have also been grouped together to make fast logic gates or high-frequency (~ 50 MHz) oscillators (for example, by IBM). CHALLENGES THAT LIE AHEAD Whereas MWCNTs are definitely making a commercial impact as structural additives in batteries and are poised to enter other markets as well, SWCNTs so far have not made it to the marketplace in substantial quantities, if at all. Part of the reason for this is that MWCNTs are really about 15 years ahead in the R&D curve over SWCNTs. It should be noted that MWCNTs really have their origins in vapor-grown carbon

Executive Summary

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fibers (VGCFs), which were first made in the late 1970s. Over time, with considerable support from the Japanese government, methods were refined to produce VGCFs at smaller and smaller diameters. SWCNTs, on the other hand, are relatively much earlier in their R&D cycle. The growth of nearly perfect micron-long SWCNTs now has been learned, and these CNTs are being used for the development of electronic devices such as FETs and chemical sensors. However, an inexpensive robust process for the bulk production of highquality SWCNTs with low fractions of amorphous carbon and residual catalyst is still under development worldwide. When this is accomplished and the cost of bulk SWCNTs drops significantly, we expect many applications to come forward that demand the higher specific surface area (cm2/gram) provided by SWCNTs. These might include EMI shielding, ESD protection, and conductive flexible transparent electrodes (in touch screens and solar cells). For the ultimate mechanical/structural applications for CNTs, it remains to be learned how to make significantly longer CNTs in bulk, e.g., hundreds of millimeters in length. When this is achieved, one would expect that the rapid development of first aerospace and then less sophisticated lightweight, high-strength components would eventually become available for general consumers. Realizing the contribution from the remarkable mechanical properties of CNTs will require further development of separation, purification, and dispersion into the host medium, and a means to covalently couple the CNTs to the host. SWCNTs have been shown to have remarkable electronic properties, e.g., high electronic mobility, ballistic transport at cryogenic temperatures, and a high tolerance to electromigration. In the area of single electronic CNT device applications, it seems to the panel that the most important challenge on the horizon is the production of a truly monodisperse set of SWCNTs, i.e., the production of a gram of material with a predominance of only one type (n,m). The ideal way to achieve this goal would be to learn how to design a catalyst particle that would only produce one chirality. This most likely will occur at a small CNT diameter (d